How to execute shell command before compile task? - scala

Question

I'd like to execute a shell command - rm -r a directory - whenever my sbt project builds. This would be before compile.
Reasoning: There's a cache file that never gets updated. If I delete it before each compile, it forces the update.
Please advise.

Answer 1

You can create a task that deletes the file:
val removeCacheTask = TaskKey[Unit]("removeCacheFile", "Deletes a cache file")
val removeCacheSettings = removeCacheTask := {
import sys.process._
Seq("rm", "/path/to/file") !
}
Then require that the task be run before compilation by adding these settings to your project:
Project(...).settings(
removeCacheSettings,
compile in Compile <<= (compile in Compile).dependsOn(removeCacheTask)
)
Source: https://groups.google.com/forum/#!topic/play-framework/4DMWSTNM4kQ
In build.sbt it would look like this:
lazy val removeCacheTask = TaskKey[Unit]("removeCacheFile", "Deletes a cache file")
removeCacheTask := {
import sys.process._
Seq("rm", "/path/to/file")!
}
compile in Compile <<= (compile in Compile).dependsOn(removeCacheTask)

Answer 2

#LimbSoup answer is fine, but there're some improvements to consider.
The command to execute rm might not be available on other non-rm OSes, like Windows, so it's much safer to use sbt.IO.delete method.
Since it's about deleting files, there's the clean task that relies on def doClean(clean: Seq[File], preserve: Seq[File]): Unit method (from sbt.Defaults) and with proper changes it could also be of some help. I think it would require a new config, though.
See How to exclude files in a custom clean task? for some guidance regarding a custom clean task.